The Flame Cell of Rotifer

The protonephridial systems of a number of metazoa consist, in part, of flame cells or flame bulbs which are specialized structures attached to paired collecting tubules. In rotifers, each tubule is said to bear two to eight (1) or more (2) flame bulbs. These structures are located in the pseu-docoelomic cavity and are believed to function in the control of osmotic pressure and, possibly, in the excretion of metabolic wastes. The cilia of the flame bulb probably create some fluid current, assisting the movement of waste liquid toward the bladder (2). The observations to be presented in this communication were made in the course of other studies in which the materials and methods have been presented in detail (3). A cross-section of a flame cell, near the level of origin of the cilia, is seen in Fig. 1. The lower portion is cut somewhat deeper (distal to the collecting tubule) than the upper portion. The flame cell is surrounded by an ill defined, external coating which is a lightly staining region approximately 200 A thick. Internal to this is a densely staining flame cell membrane which is seen (proceeding counterclockwise from the bottom) to invaginate periodically, leaving a series of empty spaces. Continuing counterclockwise, one observes that the membrane which reflects from the cilia fuses with the invaginated membrane, resulting in a series of roughly rail-shaped structures (in cross-section) surrounding the flame cell lumen, the spaces between the "rails" being continuous with the flame cell lumen (also Fig. 2). The 200-A-thick, external coating is the only material, at this level, seen separating the flame cell lumen from the pseudocoelomic cavity. The "rail-like" structures thus ringing the lumen have in their interior portion a very densely staining rodlike body (Fig. 2) which can be traced back to the level of the ciliary bases (Figs. 1 and 2). Their ultimate origin has not been traced, but, as may be seen in Figs. 3 and 4, the dense rods which alternate with the empty spaces between the "rails" appear to originate at the level of the kinetosome. These rods (Fig. 5) consist of a periodic structure of about 605 A spacing which is similar to the 640-A spacing of the ciliary rootlet structures observed by us in the coronal cilia (Fig. 6). The FIGURE 1 A cross-section through the base of a flame cell. FIGURE 6 A section showing coronal cilia (upper part …